NEW YORK (Reuters Health) - Kids hospitalized with the flu are more likely to need intensive care and a longer stay if they've been exposed to second-hand smoke at home, a small new study finds.

Analyzing the records of more than 100 kids hospitalized with flu in New York state, researchers found those exposed to second-hand smoke were five times more likely to be admitted to the intensive care unit and required a 70 percent longer stay in the hospital, compared to the kids not exposed to smoke.

"People are being a bit complacent and thinking that because they don't see smoking as often...that it's not a problem anymore," said Dr. Karen Wilson, of Children's Hospital Colorado, in Aurora, who led the study. "But we still need to be vigilant about protecting kids from second-hand smoke."

According to the Centers for Disease Control and Prevention, passive smoking causes ear infection, breathing problems and lung infections in children, and leads to the hospitalization of up to 15,000 children under the age of 18 months every year.

The new work is the first study to look at the effect of second-hand smoke on kids with influenza, however.

Wilson and her team looked at hospital records for 117 kids admitted for influenza to a New York hospital between 2002 and 2009.

Second-hand smoke exposure was reported on the charts of 40 percent of the kids - slightly lower than the 53 percent national exposure rate for kids under 11 estimated by the CDC in 2008.

During the seven-year study, researchers found that overall, 18 percent of the flu-affected kids were admitted to intensive care, and six percent needed to be intubated with a breathing tube. On average, kids stayed in the hospital for two days.

When Wilson and her team compared the kids who had been exposed to second-hand smoke to those who weren't, they found that 30 percent of smoke-exposed kids needed intensive care versus 10 percent of unexposed kids. Intubation was required for 13 percent of smoke-exposed kids, compared to one percent of those from a smoke-free home.

Hospital stays were up to 70 percent longer for smoke-exposed kids, with kids staying in for four days on average, compared with 2.4 days in non-exposed kids. If kids had a chronic illness as well as the flu, their length of stay increased to about 10 days, on average, if they had been breathing second-hand smoke, versus about three days in non-exposed sick kids.

"We've known that (second-hand smoke) is bad for children in a whole variety of ways," said Dr. Susan Coffin, who has studied flu complications in children at the Children's Hospital of Philadelphia. "With this (study) we see that smoke exposure not only increases risk of hospitalization but it specifically makes the course of illness worse."

The small study, published in The Journal of Pediatrics, does have limitations. The authors note in their report, for instance, that children with severe illnesses may have been screened more frequently for smoke exposure, leading to an underestimate of how many kids were exposed to smoke.

Still, the findings do point to a need for better screening when kids with the flu are seen in the ER, researchers said.

"If you have a child who comes into the hospital and they are exposed to tobacco smoke, they have more risk of going on to develop more severe illness," Wilson told Reuters Health. Knowing that kids are at increased risk could help physicians make better treatment decisions, she added.

For Wilson, it's critical that children don't end up in the ER in the first place.

"This is a preventable cause of severe flu, and it's sad that children are in a position to be exposed even though these serious complications can occur," said Wilson.

"Obviously not smoking and protecting children from smoke won't stop them from getting influenza, but it may help it from becoming a severe illness or (preventing) complications that we sometimes see," she added.